This invention relates generally to improvements in spinal implants of the type designed for human implantation between adjacent spinal vertebrae. More particularly, this invention relates to an improved spinal implant prosthesis including a pair of implant components defining inter-engaged articulatory surfaces having a generally elliptical profile shape and designed to accommodate limited and/or controlled articulatory movement.
Spinal implant devices are generally known in the art for surgical implantation between adjacent bony vertebral structures to correct or alleviate a number of clinical problems, such as degenerative disc disease, chronic back pain, spondylolisthesis, and others. In general terms, such spinal implants comprise a biocompatible construct formed from a relative high strength material such as titanium or cobalt chrome metal alloy, or a selected high strength ceramic, with a size and shape for intervertebral placement. In some designs, the implant has a porous region or regions adapted to receive autogenous or allogenous bone material, or otherwise defining a so-called porous bone ingrowth surface, for promoting bone ingrowth attachment of the implant device to the adjacent overlying and underlying vertebral structures. Such spinal implants are commonly used as fusion devices forming a substantially rigid interface between adjacent vertebral structures for alleviating specific patient symptoms. Alternative spinal implant designs comprise articulatory devices including upper and lower components adapted for respective fixation to overlying and underlying vertebral structures while defining an articulatory interface therebetween to maintain or restore normal or substantially normal patient movements.
A variety of articulatory spinal implant devices are known in the art. By way of example, U.S. Pat. No. 5,401,269 discloses one articulatory implant device having inter-engaged concave and convex surfaces defined by curved radial or part-circular arches, wherein the radius of the curved arches is greater in the anterior-posterior direction relative to a smaller arch radius in the medial-lateral direction. These different sizes of the inter-engaged arches in the anterior-posterior vs. medial-lateral directions beneficially accommodates a limited range of axial rotation between the articulatory surfaces, while producing counteracting forces that tend to return the engaged surfaces toward a normal neutral or non-rotated position. In addition, the engaged arched surfaces further accommodate a range of angular displacement. However, such angular displacement, e.g., in the anterior-posterior direction, or in the medial-lateral direction, is not accompanied by a counteracting or re-centering force tending to urge the engaged surfaces back toward a normal neutral position. Physical stops are provided to prevent excessive angular displacement between the implant components.
U.S. Pat. No. 6,113,637 discloses another articulatory spinal implant device having inter-engaged concave and convex surfaces of generally part-spherical or hemi-spherical shape, with the concave surface further including a centrally located cylindrical segment. This articulatory interface is designed to accommodate substantially unrestrained angular displacement as well as axial rotation between the engaged implant components, while additionally and beneficially permitting a range of fore-aft or anterior-posterior translation. Counteracting forces tending to resist over-displacement and tending to re-center the articulatory surfaces substantially in a neutral position are not present.
There exists a need for further improvements in and to articulatory spinal implant devices, particularly wherein a range of angular displacement and/or axial rotation between engaged articulatory surfaces is accompanied by counteracting forces urging return movement of the engaged surfaces substantially to an initial or neutral position, and further wherein the engaged articulatory surfaces accommodate a range of anterior-posterior displacement. The present invention fulfills these needs and provides further related advantages.